The present invention relates, in general, to the field of laser based rangefinding instruments. More particularly, the present invention relates to a non-saturating receiver design and clamping structure for high power laser rangefinders of especial utility in conjunction with monoblock laser based designs.
The United States Army Night Vision and Electronic Systems Directorate (NVESD) has developed pumped, solid state laser based devices that emit eyesafe, electromagnetic radiation in the infrared spectra for, inter alia, determining the range to a target. Such devices are an important component in the acquisition of accurate targeting information.
However, due to the applications in which such devices are employed, it is vital that they be made as small, low cost, light weight and efficient as possible. Because the most expensive part of such a system is the laser transmitter, NVESD has developed what is known as a “monoblock” laser comprising precision cut and coated crystals in the form of a laser resonator. An exemplary monoblock laser is described and illustrated in: Shilling, B. W. et al.; “End-Pumped Monoblock Laser for Eyesafe Targeting Systems”, US Army RDECOM CERDEC, NVESD, Ft. Belvoir, Va. available at: http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA480960.
Additional description of pumped lasers is given in U.S. Pat. No. 7,039,087 for “End Pumped Slab Laser Cavity” issued May 2, 2006 and United States Patent Application Publication 2010/0189140 for “Laser Diode End-Pumped Monoblock Laser” published Jul. 29, 2010. The disclosures of the foregoing paper and patent documents are herein specifically incorporated by this reference in their entirety.
Laser rangefinding devices incorporating such pumped lasers emit pulsed signals of approximately 6.0 or more millijoules with pulse widths of about 8.0 nanoseconds which corresponds to a peak power output of 750 kilowatts. This corresponds to conventional diode-based laser rangefinders which have a peak power output of from about 10 to 50 watts. The difficulty with such high powered pumped laser devices is that the returned signal easily saturates the associated receiver. For example, reflecting such a pumped laser signal off a blade of grass 10 meters away can result in a return signal of 50 milliamps which is 1,000,000 times the receiver detection threshold of 50 nanoamps causing severe saturation problems.
What is required then is a receiver design and clamping structure wherein the photodiode is capable of absorbing a massive hit of energy and yet the whole system recovers very quickly to enable the detection of a subsequent target. As an example, if the detection threshold of the photodiode is 50 nanoamps a rapid overload recovery is needed from a signal 106 times greater or 50 ma. Currently, conventional monoblock laser receiver designs are extremely complicated, incorporating multi-stacked hybrid devices which are concomitantly expensive to design and produce. Current receivers have had to incorporate variable gain circuits, clamping systems and other techniques in an attempt to solve the inherent saturation issues.